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Mitford sister hosts celebrated designer Mary X perhaps (7,8)

Ross

I believe the answer is:

unknown quantity

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'mitford sister hosts celebrated designer' is the definition.
I can't tell whether this defines the answer.

'mary x perhaps' is the wordplay.
'mary' becomes 'unknown' (I can't explain this - if you can you should give a lot more credence to this answer).
'x perhaps' becomes 'quantity' (x is a kind of quantity).
'unknown'+'quantity'='UNKNOWN QUANTITY'

Can you help me to learn more?

(Other definitions for unknown quantity that I've seen before include "Candidate not yet examined" , "any one of the last three characters?" , "Dark horse" , "one of those following VW" , "may be last in Derby, literally" .)

I've seen this clue in The Daily Mail.
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